In recent years there has been growing interest in organic electronic (OE) devices, for example organic field effect transistors (OFET) for use in backplanes of display devices or logic capable circuits, and organic photovoltaic (OPV) devices. A conventional OFET comprises a gate electrode, a gate insulator layer made of a dielectric material (also referred to as “dielectric” or “gate dielectric”), source and drain electrodes, a semiconducting layer made of an organic semiconductor (OSC) material, and often a passivation layer on top of the aforementioned layers to protect them against environmental influence and/or against damage from subsequent device manufacturing steps.
For OFET devices solution processable OSC layers are especially desired. The OSC materials should be solution based and should be suitable for solution based deposition methods like spin-coating, slot-die coating and doctor blade coating, or wider area printing methods such as flexo, gravure, and screen printing. Key requirements for OSC materials used in such solution processed layers are orthogonality of the OSC solvent towards the underlying layer, and good adhesion of the OSC to the underlying layer and to the layer deposited on top of the OSC layer.
Polymeric binders are commonly used with solution processed OSCs to increase performance of Organic Field Effect Transistors (OFET), as disclosed for example in J. Mater. Chem., 2008, 18, 3230-3236, or Appl. Phys. Lett., 2009, 94, 013506-013506-3, or to increase processability of the OSC formulation, as disclosed for example in US 2004/0038459 A1.
However, OSC formulations containing the commonly used polymeric binders, such as polystyrene, poly(4-methylstyrene), poly(α-methylstyrene), poly(4-vinylbiphenyl), and poly(triarylamine) for example, have some drawbacks in certain applications, like for example weak adhesion to available top coatings such as gate dielectrics or passivation layers.
Currently some of the most commonly used top gate dielectrics for OFETs are materials having low permittivity (also known as relative permittivity or dielectric constant) of less than 3.0 (“low k material”). The use of such low k materials is reported to reduce charge trapping at the dielectric/OSC interface and to give improved transistor performance, as disclosed for example in Adv. Funct. Mat., 2003, 13, 199-204, or in U.S. Pat. No. 7,029,945 B2.
Typical examples of such low-k dielectrics, which are orthogonal to many OSCs are the following:
1) Solution processable fluorinated materials, like those of the Teflon AF™ (DuPont) or Cytop™ (AGJ) series, as disclosed for example in US 2011/0037064 A1 or in Adv. Mat., 2009, 21, 1166-1171.
2) chemical vapor deposited poly(p-xylylene), also known as “Parylene”, as disclosed for example in Appl. Phys. Lett., 2008, 93, 183305 1-3.
In addition, in order to improve the mechanical integrity within the stack of layers present in OE devices, for example in bottom gate OFET devices, passivation layers are commonly applied on top of OSC. The passivation layers have to meet the same orthogonality and adhesion requirements as the abovementioned dielectric layers in top gate configuration.
Parylene (poly(p-xylylene) is a dielectric material that is often used in the dielectric or passivation layers of OE devices including OFETs. Parylene is a commercially available dielectric which polymerizes when being deposited on a substrate from a vaporized p-xylylene monomer. The advantage of Parylene is its relatively simple processing, resulting in a highly pure and homogeneous thin film, which is an excellent chemical barrier, both thermally stable and UV stable.
However, the adhesion of Parylene to the commonly used binder/OSC compositions, and also to many commonly available polymers which are typically used as binders, like those listed above is often very low, in many cases lower than 0.05N, but typically below detection level.
Therefore, it is desired to have available OSC compositions that show improved adhesion to Parylene layers used as dielectric or passivation layers in OE devices.
One aim of the present invention is to provide materials for use in OSC layers which meet these requirements. Another aim is to provide improved methods of manufacturing such OSC layers. Another aim is to provide improved OE devices, in particular OFETs, comprising such improved OSC layers and dielectric or passivation layers comprising Parylene. Further aims are immediately evident to the person skilled in the art from the following description.
The inventors have found these aims can be achieved by providing an OSC formulation that comprises an adhesion promoter which improves the adhesion of the OSC to Parylene, while providing good device performance.